The Expansion Pass will grant you access to four waves of DLC as they release, and owners will also receive new Officers Academy outfits for the female and male Byleth main character when the game launches.

The DLC waves are set to add new maps, costumes, and playable characters, with each one being scheduled for release between launch day and April 2020.

Wave 1: Officers Academy outfits for female and male Byleth available Day 1 (July 26th)
Wave 2: Additional Auxiliary battle maps, helpful in-game support items, and more will be released tentatively by 31 October 2019.
Wave 3: Additional quests and costumes will be released tentatively by 31 December 2019; plus, another free update
Wave 4: Discover new story content, with additional playable characters, locations and more will be released tentatively by 30 April 2020; plus, another free update